Anodic coating of zinc base metals



Patented Apr. 10, 1934 were STATES r QFFIQE 1,953,997 AN ODIC COATING F ZINC BASE DIETALS No Drawing.

14 Claims.

This invention relates to the coating of zinc base metals by anodic action, and has for its obiect the provision of an improved method of pro ducing useful anodic coatings on such metals, as

well as the provision, as new articles of commerce, of zinc base metals so coated. By zinc base metal we mean metals and alloys composed principally of zinc.

The invention is particularly concerned with the production of anodic coatings on zinc and zinc base alloys for the purpose of protection, ornamentation, or of serving as a base for the application of paints, lacquers and the like so as to secure good adhesion of the latter. Ordinary commercial zinc (in particular, rolled or strip zinc), 'zinc base alloys such as die-casting alloys containing over 90% zinc (for example, the alloys of United States Patents Nos. 1,596,761, 1,663,215, 1,779,525, 1,852,434, 1,852,441 and 1,852,442), and wrought zinc base alloys (for example, the alloys of United States Patents Nos. 1,716,599, 1,832,653, 1,832,733 and 1,835,450) may be advantageously coated by the method of the invention. Numerous other zinc base alloys, particularly those containing small amounts of copper and/or aluminum, are likewise amenable to treatment in accordance with the invention.

Anodic coatings have heretofore been produced on metals by subjecting the metal as anode to electrolytic treatment. So far as we are aware,

however, no practical and commercially satisfactory method has heretofore been available for.

the production of useful anodic coatings on zinc base metals. As the result of an exhaustive investigation of the subject we have discovered that dense and firmly adherent anodic coatings can be produced on zinc base metals from strongly basic electrolytes. Thus, we have found that with a strongly alkaline electrolyte, having a hydroxyl ion concentration corresponding to a pH value of about 13.3 or greater, highly useful coatings can be produced on zinc anodes under appropriate conditions of voltage, current density and temperature. Based on these discoveries, our present invention involves, in its broad aspect, producing an anodic coating on a zinc base metal by subjecting the metal as anode toelectrolytic treatment in an alkaline electrolyte of high hydroiwl ion concentration. The hydroxyl ion concentration should not be so high as to dissolve as zincate all of the zinc compounds formed anodically, and preferably-the electrolyte should not contain anions that form water-soluble compounds with zinc (such as anions of acetic acid, or of strong acids like sulphuric acid and hydrochloric acid).

Application November 14, 1932, Serial No. 642,608

A satisfactory electrolyte for the practice '01 the invention is an aqueous solution of a hydroxide of an alkali metal or alkaline earth metal, for example, caustic soda, caustic potash and barium hydroxide, having a pH value of about 13.3.

In carrying out the invention, the articles of zinc or zinc base alloys to be coated are subjected to an electrolytic treatment as anode while immersed in an electrolyte of high hydroxyl ion concentration. When so treated at appropriate voltages, current densities and temperatures, the zinc articles become coated with a firm adherent layer of zinc oxide and/or hydrated zinc oxide formed by reaction of zinc and hydroxyl ions. The

coatings are dark in color, commonly blue-black, and possess a pleasing lustre and iridescence.

,The coatings ordinarily produced on zinc by corrosion in solution (e. g. alkaline solutions) are in general relatively loose and poorly adherent. In contrast with this, the anodic coatings produced in accordance with the present invention are thin, dense and firmly adherent. These anodic coatings usually reproduce in minute detail the surface markings originally present on the treated metal objects. This is probably due to the fact that the coating is formed primarily by reaction of the hydroxyl ions discharged at the anode with zinc atoms in situ in the crystalliies of the zinc metal. On the other hand, it is possible that the coating is formed in whole or in part by reaction of hydroxyl ions with zinc ions in solution in the electrolyte at the surface of the anode, followed by precipitation upon the anode of the insoluble zinc compound thus formed. The X-ray patterns of the coatings indicate that they consist essentially of zinc oxide. 1

Since the anodic coating of the invention usually reproduces, in minute detail, the surface markings originally present on the zinc article, the appearance of the coating may be improved by buffing the zinc article before anodic treatment. This buffing procedure may advantageously be of the same character as used on zinc prior to electro-plating. Other preliminary treatment, such as sand-blasting or etching which, like bufiing, removes a surface film especially likely to be present on cast zinc surfaces, may be employed to facilitate the subsequent anodic treatment. Such preliminary surface treatment of the zinc article may, of course, be

per gallon of water (45 grams per liter of water). This solution is used hot as an electro-cleaner, with the zinc article to be cleaned as the cathode, and with sufiicient current to produce copious 5 gassing. When used in this manner, grease (e. g.

acquired during rolling or bufiing) and loose dirt are completely removed after one-half to three minutes of cleaning. After cleaning, the zinc article is rinsed in hot and then in cold water to remove the residual cleaning solution, following which the zinc article is ready for anodic treatment.

A very satisfactory electrolyte for the practice of the invention is an 0.5N aqueous solution of sodium hydroxide. The electrolytic treatment is conducted at room temperature with acurrent density at the anode of from 10 to 60 amperes per square foot, and with a duration of treatment of from 1 to 15 minutes. Under these operating conditions, dark, usually blue-black, coatings are obtained on anodes of zinc and of zinc base alloys of the types hereinbefore mentioned. The coatings are firmly adherent, uniform in color and duplicate the minute markings on the original surface in every detail. The coatings may be given a high lustre by bufling. If entirely black coatings are desired, the films may be dyed with nigrosine or the like.

The specified operating conditions may be varied without departing from the spirit and scope of the invention. Thus, the concentration of the sodium hydroxide solution may be varied from 0.2N to 1.0N without changing the essential properties of the anodic deposit. The current density may likewise be varied, but the use ofcurrent densities less than 10 amperes per square foot requires an unnecessarily long time of treatment while the use of current densities above 60 amperes per square foot results in the use of an unnecessarily high current. A 6 to 12 volt source of electric energy is sufiicient. Technical flake sodium hydroxide is sufficiently pure for the purpose. The cathode used in the electrolytic treatment may be of nickel, iron or other metal insoluble in sodium hydroxide solution.

A saturated solution of barium hydroxide is also a satisfactory electrolyte for the practice of the invention. Under the same operating conditions as hereinbefore described, and with a ba-- rium hydroxide electrolyte saturated at room temperature (the operating temperature), anodic films are produced of the same character as obtained with the 0.5N sodium hydroxide solution. Barium hydroxide can be added to sodium hydroxide electrolytes without altering the nature of the anodic coatings obtained therefrom. Thus,

if it is desired to keep a sodium hydroxide electrolyte free from sodium carbonate formed by absorption of carbon dioxide from the air, barium hydroxide may be added to precipitate any soluble carbonate as barium carbonate.

During the anodic treatment, some zinc goes into solution in the strongly alkaline electrolyte. Loss in weight of the zinc anode has been determined as a function of the current density and time of treatment, and while such losses are not more than 1 or 2 grams per square foot, on the average, they are found to decrease as the current density is increased, for a given quantity of current flowing. This small amount of zinc enters solution as zincate ions, with which a low concentration of zinc ions are in equilibrium, as is shown by the fact that a small amount of zinc is deposited electrolytically at the cathode. The electrolyte actually present during the operation thus contains a small amount of zinc (e. g. sodium zincate) as well as sodium hydroxide. The presence of this zinc in solution in the electrolyte exercises no deleterious influence on the anodic treatment so long as the hydroxyl ion concentration of the electrolyte corresponds to a pH value'of about 13.3 or higher. Similarly, with this high hydroxyl ion concentration (pl-I of 13.3 or greater), the electrolyte may contain,

by contamination or otherwise, small amounts of other dissolved substances.

The anodic coatings of the invention not only protect the zinc or zinc alloy from corrosion, but are also useful for decoration. These coatings may also be used as a base to increase the adherence of paint, enamel, lacquer, etc. Anodic coatings produced in accordance with the invention, although composed for the 'most part, if not entirely, of zinc oxide, are dark, commonly blue-black in color. The coating may be dyed jet black by nigrosine, or similar dye. The dyeing may take place during the electrolytic treatment by solution of the dye in the electrolyte, or dyeing may be carried out by immersion of the coated article in a solution of the dye.

We claim:

1. As a new article of commerce, a zinc'base 3. The method of producing a coating on azinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte having a pI-I of about 13.3 or

greater.

4. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte having a hydroxyl ion concentration greater than the equivalent of a 0.2N sodium hydroxide solution.

5. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte having a concentration of free alkali not less than the equivalent of a 0.2N sodium hydroxide solution.

6. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkaline electrolyte with a high hydroxyl ion concentration that is adapted to produce an anodic coating on the zinc metal that is dark in color, firmly adherent and composed for the most part of zinc oxide.

7. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkalin electrolyte with a high hydroxyl ion concentration that is adapted to produce an anodic coating on the zinc metal that is blue-black in color, firmly adherent and composed for the most part of zinc oxide.

8. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment at a current density of from 10 to 60 amperes per square foot of anode surface in an alkaline electrolyte having a pH of about 13.3 or greater.

9. The method of producing a coating on a zinc base metal comprises subjecting the metal as anode to electrolytic treatment for from 1 to 15 minutes at a current density of from 10 to 60 amperes per square foot in an alkaline electrolyte having a pH of about 13.3 or greater.

10. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in a 0.2N to 1.0N solution of sodium hydroxide.

11. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in a solution of sodium hydroxide having'a concentration of about 0.5N NaOH.

12. The method of producing a coating on a zincbase metal which comprises subjecting the metal as anode to electrolytic treatment at a current density of from 10 to 60 amperes per square foot of anode surface in a 0.2N to LON solution of sodium hydroxide.

13. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in a substantially saturated solution of barium hydroxide.

14. The method of producing a coating on a zinc base metal which comprises subjecting the metal as anode to electrolytic treatment in an alkalin electrolyte containing sodium hydroxide and barium hydroxide and having a pH of about 13.3 or greater.

EDWARD CUSHMAN 'I'RUESDALE. ERNEST JOHN WILHELM. 

